For certain high speed and/or high powered integrated circuit applications, silicon on insulator (SOI) chips are desirable. SOI chips are typically fabricated from wafers that have a layer of oxide sandwiched between an active layer of silicon formed on the top surface of the wafer and a bulk layer of silicon formed on the bottom surface of the wafer. Transistors, electrical components, interconnect, and the like, are formed either within or on the active surface of the wafer. The bulk layer is provided to add mechanical strength or rigidity to the wafer.
SOI wafers can be formed using a number of well known techniques. According to one technique, a layer of oxide is grown over the surface of the bulk silicon layer. The active layer is then bonded over the oxide layer. In another method called SIMOX (Separation by Implantation of Oxygen), oxygen is implanted into the silicon at an energy level sufficient to form the oxide layer in the silicon wafer. For more information on this technique, see “SIMOX (Separation by Implantation of Oxygen”, by Julian Blake, Encyclopedia of Physical Science and Technology, Jul. 28, 2001, incorporated by reference herein.
Generally speaking, CMOS transistors fabricated on SOI chips are used for high speed applications. Bipolar transistors on SOI chips are used for high power applications. In either case, trench like recess regions, filled with oxide, are formed around the four sides of each transistor. The oxide typically extends the entire depth of the active layer and contacts the oxide underlying layer formed over the bulk silicon layer. As a result, a complete isolation structure is encapsulated around and underneath the transistor. Electrically isolated transistors can be placed closer to one another than transistors without the isolation. Consequently, the circuit density can be increased.
Heat dissipation is a significant problem with SOI chips. Oxide is a relatively poor heat conductor. High speed and/or high powered transistors tend to generate a great deal of heat during operation. Since the aforementioned transistors act as a heat source and are surrounded by insulation, (the oxide layer) the temperature of the active layer can significantly increase. In severe situations, the switching characteristics of the transistors may be adversely affected, causing the circuitry to not operate properly or fail.
An apparatus and method of providing a heat sink to dissipate the heat sourced by the encapsulated transistors of a SOI chip, is therefore needed.